14 Jul Smell the summer!
What smells make you think of summer? If you say: sunscreen, freshly cut grass, summer rain or some tropical fruit, you’re certainly not the only one. But why do these odours smell so good?
Scientists at the Rockefeller University (New York) found out that the human nose can detect one trillion different odours. This suggests that the human nose outperforms the eye and ear in terms of number of stimuli it can distinguish between. Oh, and there’s twelve zeros in one trillion.
"Smell is a potent wizard that transports you across thousands of miles and all the years you have lived"~Helen Keller~
According to Psychologist Pamela Dalton it’s not the smell itself that we like, but the association with our experiences. So, we like the smell of sunscreen, grass and summer rain because it reminds us of summer and weekends. For the complex smell of summer rain we even thought up a name: petrichor.
The smell of summer rains
Petrichor is the scent produced when rain falls on dry soil. The term was first used by two researchers who published an article in Nature in 1964. They describe how the smell derives from a pair of chemical reactions. Some plants secrete oils1 during dry periods. In turn, these are absorbed by the soil. When it rains, the oils are released into the air. A second part of the odour is the aroma of geosmin, a metabolic by-product of certain soil-dwelling bacteria. This makes up the distinct damp-earth smell. The combination of the oils and geosmin creates the pleasant petrichor scent.
Before the rain begins, one of the first odours you may notice is a pungent smell, reminiscent of chlorine. That is the sharp and fresh aroma of ozone, 03. The highest level of ozone is in the stratosphere, that’s between 10 and 50 km above the Earth’s surface. Most clouds are below the ozone layer in the troposphere. The scent of ozone announces stormy weather, as the downdrafts of thunderstorms carry ozone from these high altitudes.
The odour of green leaves
When you smell freshly cut grass, you’re actually smelling a mixture of oxygenated hydrocarbons called green leaf volatiles. They include methanol, ethanol, acetaldehyde and acetone. And the smell they produce isn’t just an accidental by-product of the cut leaves. Plants react to mechanical damage to their leaves by emitting these volatiles as a signal. For instance, cis-3-Hexen-1-ol, also known as leaf alcohol, is a volatile that is produced by most plants and attracts predatory insects. Wild tobacco plants only emit a specific green leaf volatile when they are grazed by caterpillars. Again, the volatile acts as a signal to attract nearby bugs that pray on the caterpillars. Research towards these signals is also done at our faculty by Merijn Kant and Robert Schuurink.
Did you ever smell a thunderstorm coming your way? Or do you have very different favourite summer scents? Don’t forget to comment!
1) The oils have a suppressive effect on the germination of seeds and early plant growth. Possibly to protect young plants during dry spells in the summer.